There is conventionally known an electro-optical element as an optical element using an electro-optical effect. The electro-optical element is used for optical intensity modulators, Q-switch elements, light beam scanners or the like because the electro-optical element has a property that a response to change in electric field is very rapid.
The electro-optical effect means, in general, physical phenomenon that when electric field is applied to an object through which light passes, a refractive index thereof changes. In addition, an effect that a refractive index changes in proportion to intensity of an electric field is referred to as Pockels effect. An amount of change in a refractive index is given by the following equation (1).Δn∝rij×V/d  (1)
Where rij is electro-optical constant (Pockels constant), V is an applied voltage, and d is a space between electrodes applying a voltage.
An amount of change in the refractive index caused by the electro-optical effect is relatively small, for example, if lithium niobate which is a typical electro-optical material is used, it is necessary to apply a large electric field of about 6 kV/mm to an electro-optical crystal (EO crystal) in order to give change in a refractive index of about Δn=0.001. Practically, it is requested that the electro-optical element can operate with a low voltage and has good response characteristic with a wide frequency band. It is effective that a space d between electrodes is set to be a small value as is clear from the equation (1) to acquire large change of refractive index with a low voltage.
On the other hand, as a typical example of forming an optical waveguide in an electro-optical material, there is known a technology of forming the optical waveguide by impurity diffusion, in which titan is locally diffused in an electro-optical material of lithium niobate or the like, and a refractive index of the titan-diffused area only is increased, thereby the optical waveguide is formed. In the optical waveguide structure, electrodes are provided adjacent to an optical waveguide area formed by the impurity diffusion on a surface of the electro-optical crystal.
As another example of forming the optical waveguide in the electro-optical material, there is known a technology of forming the optical waveguide, in which an electro-optical crystal is thinned by polishing or the like, and electrode layers are provided on an upper surface and a lower surface of a substrate as a core layer, formed by the thinned electro-optical crystal, thereby the optical waveguide is formed. In this case, a direction of applying an electric field is perpendicular to the upper surface and the lower surface of the substrate of the electro-optical crystal. In the technology of forming the optical waveguide in an electro-optical element, because it is possible to form the optical waveguide structure while maintaining a characteristic of the electro-optical effect which is innate characteristic of the electro-optical crystal, an electro-optical element effective to a low voltage operation can be provided.
In such a thin-film type-electro-optical element, it is necessary to provide as a clad layer a transparent material having a refractive index lower than that of the core layer between the thinned substrate and each electrode, in order to propagate waveguide light while confining the light in the electro-optical crystal substrate of a thinned film as the core layer of the waveguide. As a typical clad material forming the clad layer, a dielectric material such as SiO2 is used.
If a voltage is applied to the optical waveguide formed by the core layer and the clad layers, the voltage applied to the core layer is obtained by the following equation (2).Vco=V/[1+(dcl/dco·∈co/∈cl)]  (2)
Where V is a voltage applied to the optical waveguide, Vco is a voltage applied to the core layer, dco is a thickness of the core layer, dcl is a thickness of each of the clad layers, ∈co is a dielectric permittivity of the core layer, and Ecl is a dielectric permittivity of each of the clad layers.
As is clear from the equation (2), when the clad layers are provided, because denominator of right-hand side of the equation (2) is larger than 1, a voltage applied to the electro-optical crystal substrate itself, that is to say, a voltage Vco applied to the core layer becomes a low value.
For example, in an optical waveguide type-electro-optical element in which a core layer is formed by a thin film made of lithium niobate, and each of clad layers is made of SiO2, if the core layer has 10 micrometers in thickness, and each of the clad layers has 1 micrometer in thickness, a voltage applied to an electro-optical crystal itself (lithium niobate of the electro-optical material) is only about 0.4 times as much as a voltage applied to the entire electro-optical element, and therefore the electro-optical element itself has a high operational voltage.
Consequently, to apply a high voltage to the electro-optical material, it is necessary to reduce a thickness of each of the clad layers. For example, if each of the clad layers is thinned to be about 0.2 micrometers, a voltage applied to the electro-optical material is about 0.8 times as much as a voltage applied to the electro-optical element. This is practical if it is considered that an operational voltage is reduced. In this way, in a conventional optical waveguide type-electro-optical element, it is effective for a low voltage driving to thin the clad layers if possible (for reference, see Japanese Patent Application Publication No. 2009-080378 which is referred hereinafter to as Patent Document 1).
In the Patent Document 1, there is disclosed a structure in which an electro-optical element includes a core layer made of strong dielectric material and having a substrate of silicon, and clad layers comprising thin films each having a high dielectric permittivity in order to inhibit voltage drop in the clad layers, and a method for manufacturing the clad layers.
In the technology disclosed in the Patent Document 1, because each of the clad layers has a thin thickness, when a high electric field is applied to the electro-optical element, there is a problem that insulation breakdown occurs in the clad layers. In other words, when the electro-optical element is operated by applying a voltage of a high-intensity electric field to the electro-optical element, there is a problem that insulation breakdown occurs. If a voltage is applied to the electro-optical element, an electric field intensity applied to the clad layers is given by the following equation (3).Ecl=∈co/∈cl·Eco  (3)
Where Ecl is an electric field-intensity applied to each of the clad layers, and Eco is an electric field-intensity applied to the core layer.
In an electro-optical element in which if the core layer is formed by a thin film made of lithium niobate which is one of an electro-optical material, and each of the clad layers is made of SiO2, for example, ∈co/Ecl is is about 7, an electric field which is 7 times higher than the electric field of the electro-optical element is applied to the clad layers. As an example, in an electro-optical element in which the core layer is formed by a thin film having 10 micrometers in thickness made of lithium niobate, and the voltage of 100V is applied to the electro-optical element, an electric field intensity applied to the core layer is 10 kV/mm, and the high electric field, 70 kV/mm is applied to the clad layers.
Because a voltage of insulation breakdown in a general silica glass is about 40 kV/mm, there is high possibility that insulation breakdown occurs in the clad layers. If the insulation breakdown occurs in the clad layers, the electro-optical element is subject to adverse effects as mentioned hereinafter.
First, it is considered that the insulation breakdown does not occur in the entire area of the clad layers simultaneously, but occurs locally in a partial area of the clad layers, for reasons of a thickness distribution of a film of each of the clad layers when the film is formed, a surface roughness of the film, adhesion between the film and the core layer, or the like.
In a place at which the insulation breakdown occurs, because each of the clad layers which are essential insulators functions as a conductor, if a small insulation breakdown occurs in the clad layers, it is considered that the same operation as a case where one or more small protrusions are provided on electrodes are performed in the place of the insulation breakdown.
It is known that an electric field is concentrated on the small protrusions. Therefore, there is possibility that insulation breakdown occurs in the core layer made of an electro-optical material. Even if insulation breakdown does not occur in the core layer, a conductive area is formed in the clad layers, and thereby there is possibility that charges from electrodes are injected in the core layer through the conductive area.
Even in cases where insulation breakdown occurs locally in the core layer of the electro-optical material and charges are injected in the core layer through the clad layers, the charges are unevenly injected in the core layer of the electro-optical material. When the charge injection occurs, an electric field formed in the core layer is broken by the charge injection. Consequently, when the electro-optical element is operated, inhomogeneous electric field intensity is formed in the core layer by the local charge injection.
When the electro-optical element is operated, it is important that an electric field is evenly formed in the core layer. In particular, if electric field within the optical waveguide is inhomogeneous, an amount of change of refractive index caused by the electric field is different every areas of the optical waveguide. Consequently, a shape of light beam propagated in the optical waveguide is significantly deformed.
In particular, in a case of a slab optical waveguide where an optical waveguide has no structure confining charges laterally, deformation of a beam shape by the inhomogeneous electric field is significantly appeared. The deformation of the beam shape results in reduction of extinction ratio, if the electro-optical element is used as a modulator, and in deterioration of a shape of emitted light beam and reduction of image dissection score which can be resolved by a scanner for light beam, if the electro-optical element is used as the scanner.
In this way, there is a problem in the conventional clad layers that a beam shape of light beam passing through the optical waveguide is deformed, and hence performance of the electro-optical element is degraded